A 51-year-old male has just been transferred to ICU from the surgical ward with worsening shortness of breath five days post-oesophagectomy, and a presumed anastomotic leak.

On arrival in ICU he is tachypnoeic and extremely agitated.

Arterial blood gas analysis on FiO2 0.6-0.8 via reservoir (non-rebreathing) mask shows:

Parameter Patient Value Normal Adult Range
pH 7.12* 7.35 – 7.45
PaO2 50 mmHg (6.6 kPa)  
PaCO2 50 mmHg (6.6 kPa)* 35 – 45 (4.6 – 6.0)
HCO3 16 mmol/L* 22 – 28

Chest X-ray shows bilateral pulmonary infiltrates.

a) List the possible causes for his respiratory failure.

The patient is intubated and mechanical ventilatory support is initiated.

b) Describe the ventilator settings you will prescribe, giving the rationale for your decision.

Following intubation, there is no immediate improvement in the patient’s oxygenation

c) List the initial strategies that may be used to improve oxygenation.

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College Answer

a) List the possible causes for his respiratory failure.

Differential diagnosis should include:
 ARDS secondary to sepsis from any source or other inflammatory insult including the following
 Pneumonia (hospital-acquired)
 Aspiration
 Atelectasis/pleural effusions/empyema
 Fluid overload secondary to resuscitation, renal failure
 Exacerbation of pre-existing condition e.g. heart failure, valvular heart disease, post-op
ischaemia/MI, arrhythmia
 Lung diseases e.g. lymphangitis carcinomatosis

The patient is intubated and mechanical ventilatory support is initiated.

b) Describe the ventilator settings you will prescribe, giving the rationale for your decision.

 Use a mode with which one is familiar and aim to limit ventilator-associated lung injury, i.e
oxygen toxicity, barotrauma, volutrauma, shear stress and biotrauma
 Choice of mode (any appropriate answer acceptable e.g. APRV for recruitment benefit, or
volume assist control as staff familiarity and no one mode shown to have benefit over another)
 Avoid over-distention of alveoli by keeping tidal volumes at 6-8 ml/kg (predicted body weight
which in the ARDSnet studies was ~20% below actual body weight and calculated by a formula
linking height and sex)
 Use PEEP to minimise alveolar collapse and derecruitment.
 Titrate PEEP to achieve a PaO2 of 60 mmHg with lowest FiO2 that is needed using decremental
PEEP trial post recruitment manoeuvre.
 I:E ratio of  1:1
 Permissive hypercapnea to avoid large minute volumes and alveolar injury through collapse and expansion of lung units

Following intubation, there is no immediate improvement in the patient’s oxygenation

c) List the initial strategies that may be used to improve oxygenation.

 High FiO2 (titrated to lowest possible level to limit toxicity)
 Confirm ETT position and patency
 Exclude readily reversible cause of hypoxia e.g. PTX, mucus plug, large effusion
 Increased inspiratory time
 Increased PEEP
 Recruitment manoeuvre with decremental PEEP trial
 Prone positioning for at least 16/24 hours per day
 Ensure adequate cardiac output

Discussion

a) List the possible causes for his respiratory failure.

In the chapter on the definition, causes and differential diagnosis of ARDS there is this table:

Differential Diagnosis for Diffuse Bilateral Pulmonary Infiltrates

Vascular:

  • Pulmonary haemorrhage
  • Cardiogenic pulonary oedema

Infectious

  • Bacterial
  • Viral
  • Fungal
  • PJP

Neoplastic

  • Lymphangitis
  • Infiltrative neoplasm

Idiopathic

  • ARDS

Drug-induced

  • Eosinophilic pneumonitis
  • BOOP
  • Alveolar haemorrhage
  • Methotrexate-induced

Autoimmune

  • Goodpastures (haemorrhagic)
  • Rheumatoid pneumonitis

Traumatic

  • Bilateral atelectasis
  • Pulmonary contusions

The next question asks the candidate to describe the ventilator settings you will prescribe, giving the rationale for your decision.  After those settings fail to work, the college asks for strategies that may be used to improve oxygenation. Overall this question is asking about  Ventilation strategies for ARDS. These are discussed in greater detail elsewhere. A brief summary may be offered:

Initial ventilator strategy:

Additional ventilator manoeuvres to improve oxygenation:

Non-ventilator adjunctive therapies for ARDS:

Ventilator strategies to manage refractory hypoxia

  • Prone ventilation, for at least 16 hours a day (PROSEVA, 2013)
  • High frequency oscillatory ventilation may not improve mortality among all-comers (OSCAR, 2013) or it may actually increase mortality (OSCILLATE, 2013) but some authors feel that there were problems with methodology.

Non-ventilator adjuncts to manage refractory hypoxia

  • Nitric oxide was a cause for some excitement, but is no longer recommended.
  • Prostacyclin is still a cause for excitement, and is still vaguely recommended.
    • Neither agent improves mortality, but prostacyclin can improve oxygenation.
  • ECMO may improve survival (CESAR, 2009) but again there were problems with methodology.

References

References

Blanco, Silvia, and Antoni Torres. "Differential Diagnosis of Pulmonary Infiltrates in ICU Patients." www.antimicrobe.org

ARDS Definition Task Force. "Acute Respiratory Distress Syndrome." Jama307.23 (2012): 2526-2533.